(1) Technical Field
This invention relates to electronic circuits, and more particularly to radio frequency attenuator circuits.
(2) Background
An attenuator is an electronic device that reduces the power of a signal without appreciably idstorting its waveform, and is widely used in electronic circuits, particularly in radio frequency (RF) circuits. An attenuator is functionally the opposite of an amplifier (although the two work by different methods)—while an amplifier provides gain, an attenuator provides loss (or, equivalently, a gain less than one). Classic examples of RF attenuators are bridged-T type, pi-type, T-type, and L-pad type attenuators.
An ideal RF attenuator would not affect the phase of an applied signal when switched into an attenuation state or into a non-attenuating reference state (also known as a bypass state). However, in actual circuit embodiments, conventional attenuators have a different phase characteristic in their attenuation and reference states. As the frequency of an RF signal applied to a conventional attenuator increases, the amount of phase shift also increases. This characteristic can be problematic if a constant phase shift is desired from the attenuator (for example, in a phase array antenna).
FIG. 1a is a schematic diagram of one embodiment of a conventional switchable pi-type RF attenuator 100. In the reference state, an RF signal applied to an In port passes to an Out port through a closed switch M0 (shown implemented as a field effect transistor, or FET), while open shunt switches M1 and M2 nominally isolate respective resistors R1 and R2 from circuit ground. In the attenuation state, switch M0 is open and an RF signal applied to the In port passes to the Out port through a resistor R0, while closed shunt switches M1 and M2 connect respective resistors R1 and R2 to circuit ground. (Since the circuit is generally symmetrical, either port can be “In” or “Out”, hence the labelling is arbitrary in this example).
While widely used in RF circuits for switching, a FET is not an ideal switch having zero impedance when closed and infinite impedance when open. However, the resistance of a closed FET, RON, is often negligible and thus can be modeled as a simple conductor in many applications. Such is not the case for an open FET, which presents as a capacitance COFF that generally cannot be ignored at RF frequencies, particularly at high RF frequencies (e.g., above about 10 GHz).
FIG. 1b is a diagram of a circuit model of the pi-type RF attenuator 100 of FIG. 1a in a reference state, with switch M0 closed and switches M1 and M2 open. A closed FET switch is modeled as a simple conductor and an open FET switch is modeled as a capacitor. The attenuator essentially becomes a low pass filter, and will induce a phase lag in any applied RF signal.
FIG. 1c is a diagram of a circuit model of the pi-type RF attenuator 100 of FIG. 1a in an attenuation state, with switch M0 open and switches M1 and M2 closed. Again, a closed FET switch is modeled as a simple conductor and an open FET switch is modeled as a capacitor. The attenuator essentially becomes a high pass filter, and will induce a phase lead in any applied RF signal.
FIG. 2 is a graph 200 showing an approximation of phase as a function of frequency for the reference state 202 and the attenuation state 204 of the pi-type RF attenuator 100 of FIG. la when an RF signal is applied. As is clear from the graph 200, switching from the reference state 202 to the attenuation state 204 causes a significant shift in the phase of the applied RF signal. A further complication is that, at high RF frequencies, ground inductance (not shown in the models of FIG. 1b or FIG. 1c) and the COFF of the M0 FET switch work to reduce the amount of attenuation that the circuit is designed to provide.
Referring again to the pi-type RF attenuator 100 of FIG. 1a, optional capacitors (not shown) can be added between the reference voltage (circuit ground in this example) and the In and Out ports to help reduce attenuation and phase changes as a function of frequency; however, doing so only works for a narrow bandwidth.
Accordingly, there is a need for a wideband RF attenuator circuit that has a reduced impact on the phase of an applied signal when switched between an attenuation state and a non-attenuating reference state. The present invention meets this need.